Pulmonary lymphangioleiomyomatosis (LAM) is a poorly understood disorder characterized by abnormal smooth muscle (SM)-like, tuberin-minus cell proliferation leading to lung destruction and pseudocyst formation. Serum response factor (SRF) is a transcription factor that plays an essential role in the regulation of SM myogenesis and cell growth. We recently found that LAM lesions exhibit high SRF expression levels. Furthermore, our studies demonstrated that upregulation of SRF in normal lung fibroblasts stimulates production of key extracellular matrix (ECM)-degrading enzymes including several metalloproteinases (MMPs) and urokinase plasminogen activator (uPA). On the contrary, their corresponding inhibitors, tissue inhibitor of metalloproteinases-3 (TIMP-3) and plasminogen activator inhibitor-1 (PAI-1) are downregulated, resulting in an enzymatic imbalance that favors ECM degradation. Immunohistochemistry and laser microcapture confirmed that this imbalance is also present in LAM lesions. Since SRF is in addition a well established mitogenic transcription factor we hypothesize that high SRF levels contribute to the pathogenesis of LAM by creating a pro-proteolvtic imbalance that favors ECM degradation and LAM cell proliferation/survival. Hence, SRF downregulation should eliminate these deleterious features. Furthermore, we hypothesize that exogenous TIMP-3 should also correct the proteolvtic imbalance in a more direct manner. To address our hypotheses we developed unique LAM culture systems, including LAM cell primary cultures (identifying LAM cells based on two novel LAM cell markers), lung explant cultures, and LAM xenografts in SCID mice. Here we specifically propose: 1- To determine whether SRFdownregulation will correct the pro-proteolytic imbalance in LAM cells. 2- To elucidate whether SRF downregulation will change LAM cell behavior with regards to ECM invasion and cell proliferation/apoptosis. 3- To determine whether exogenous TIMP-3 will decrease the activity of MMPs and improve LAM cell behavior. Our hypotheses will also be tested on ELT-3, a tuberin-minus cell line, to integrate the proposed studies with existing research on LAM. We believe that the new knowledge obtained from these studies will help to better understand the pathogenesis of LAM and to devise novel therapeutic strategies to treat this disease.